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I'm running Ubuntu 12.04 in Virtualbox 4.2.1 on the Macbook Pro Retina. For the most part it works fairly well after installing guest additions.

It freezes up at times for a minute or so then catches up with itself. (Sometimes I get a black screen for 30 seconds or so until I see Ubuntu again.)

The only other issue might be the fact that since OS X is scaling the resolution, everything in Ubuntu looks slightly blurry. It's not too problematic for general tasks, however it can definitely be an issue if you are a designer who wants pixel perfect accuracy.

There are a couple of people who have 12.10 up and running, though with a few hinges (flaky wifi, broken brightness control, no graphic switching). More information, including guides, screenshots, pitfalls: